Long-term LGBT and human rights activist Tatchell was part of campaigns in the early 1990s which threatened to ‘out’ 200 clergy, politicians and other public figures who opposed LGBT equality but were secretly gay or bi themselves.

Tatchell told GSN he knew of ‘half a dozen’ clergy and politicians who are speaking against gay and lesbian marriage equality despite their hidden sexuality.

He told us he was ‘considering’ whether to out them. He wants his comments to us to be seen as a warning shot – if they don’t back away from their opposition, he may feel morally obliged to act.

He said: ‘There is a strong ethical case for exposing public officials who abuse their power to harm other gay people by supporting homophobic laws.’

Tatchell’s possible actions were also alluded to in a statement sent out to a range of media this afternoon. In it, he gave his response to O’Brien’s resignation.

While the allegations against O’Brien have not been investigated and remain unproven, he was accused of ‘inappropriate’ contact with four priests, one of who has now left the clergy and married, allegedly traumatized by what happened.

Tatchell said: ‘Cardinal O'Brien condemned homosexuality as a grave sin and was a long-time opponent of gay equality. He supported homophobic discrimination in law, including the current ban on same-sex marriage.

‘In the light of these allegations, his stance looks hypocritical. He appears to have preached one thing in public while doing something different in private.

‘Several other prominent opponents of equal marriage are guilty of double standards and vulnerable to similar exposure.’

The nature of the allegations against O’Brien is different, however, as his some of his accusers suggest the ‘inappropriate’ approaches and behavior were not welcome.

Tatchell said: ‘We don’t know the full facts yet but the published allegations allude to the possibility of not only consensual actions but abuse or harassment.’

Tatchell also attacked the Catholic hierarchy more generally for their stance on LGBT issues.

He said: ‘It is estimated that around 40% of Catholic priests in Britain are gay, which makes the church's opposition to gay equality so two-faced and absurd.

‘A significant proportion of all Cardinals worldwide is thought to be gay.

‘Recent revelations in Italy have alleged the existence of a gay mafia within the Vatican, including senior Cardinals and other Vatican officials, and their participation in gay bars, clubs, saunas, chat rooms and male prostitution services.

‘The Vatican is shamelessly championing homophobia and the denial of legal equality to gay people, while hosting a hotbed of secret, guilt-ridden clerical homosexuality.’

In other responses to O’Brien’s departure, Ben Summerskill, chief executive of gay campaign organization Stonewall, called for a full investigations into the ‘serious’ allegations.

Stonewall named O’Brien as their Bigot of the Year in their annual awards in November 2012.

Summerskill added: ‘We hope his successor will show a little more Christian charity towards openly gay people than the Cardinal did himself.’

And Tom French, policy coordinator for the Equality Network, an LGBT campaign organization in Scotland, did not comment on the allegations against O’Brien but said it was an opportunity for the Catholic Church to rethink its homophobic and transphobic stance.

‘The Catholic Church does a huge amount of good work on issues like poverty, and it’s a shame that this important work is so often overshadowed by its position on issues of sexuality,’ he said.